Security systems are limited in their ability to detect contraband, weapons, explosives, and other dangerous objects concealed under a person's clothing or in an object, such as a box or bag. Metal detectors and chemical sniffers are commonly used for the detection of large metal objects and some kinds of explosives, however, a wide range of dangerous objects exist that cannot be detected with these devices. Plastic and ceramic weapons developed by modern technology increase the types of non-metallic objects that security personnel are required to detect. The alternative of manual searching of subjects is slow, inconvenient, and would not be well tolerated by the general public, especially as a standard procedure in, for example, airports.
Radiation exposure is an important consideration in x-ray concealed object detection systems. The United States National Council on Radiation Protection (NCRP), in NCRP Report No. 91, “Recommendations on Limits for Exposure to Ionizing Radiation”, 1987, addresses this issue. In this report, the NCRP states that a radiation exposure of less than 1000 microRem per year in excess of environmental levels is negligible, and efforts are not warranted at reducing the level further. Persons employed in high security or secured facilities, or those who frequently travel by airlines, may be subjected to many hundred security examinations per year. A yearly radiation exposure limit of 1000 microRem safely permits a single scan exposure within the range of 1 to 10 microRem for the general public. In accordance with the NCRP recommendations, radiation levels significantly higher than this may present some health risk.
Known prior art x-ray systems have limitations in their design and method which prohibit them from achieving the low dose and high image quality that are prerequisites to commercial acceptance. For example, radiant energy imaging systems that detect concealed objects carried on or in an object often scan pencil beam of x-rays through the object where the beam is transmitted or absorbed depending upon the concealed object, if any. A detector may be scanned vertically behind the object in step with the pencil beam to collect the transmitted x-rays.
U.S. Pat. No. 5,181,234 (the '234 patent), herein incorporated by reference as if set forth fully herein, discloses an imaging system which does not require x-rays to be scanned through the object. The '234 patent discloses an imaging apparatus where a narrow pencil beam of x-ray radiation is scanned over the object whereby x-rays that strike low atomic number materials, such as soft tissue, are scattered (i.e. reflected) back toward the apparatus. In comparison, x-rays that strike metal are mostly absorbed and generate very little scatter. Moreover, x-rays that do not strike the object are not captured or scattered back toward the apparatus since the x-rays continue until absorbed or scattered by items further behind the object. Detectors within the apparatus capture the scattered x-rays and generate a corresponding image. For example, as shown in FIG. 1, the vast majority of the body 12 appears light, as a result of the soft tissue generating significant back scatter of x-rays. Metals such as coins in the pocket 102 and belt buckle 103 appear dark due to their absorption of the x-rays. The background 104 around the body is also dark since there is nothing to scatter the x-rays back to the detector.
As shown in FIG. 2, a potential disadvantage of this approach is the difficulty in detecting metal objects that appear in front of or against the background 104 and not in front of the body 12. FIG. 2 shows a metal handgun 106 concealed under the subject's 12 arm. The handgun 106 is virtually impossible to detect in this view since both the handgun 106 and the background 104 appear dark. People may also hang or wear contraband on their sleeves or pant legs, which would be difficult to detect since it would be against the background and appear dark.
Another potential disadvantage of the prior art is that it provides no mechanism to control the x-rays not scattered or absorbed by the object. Currently, the x-rays not scattered or absorbed by the object continue until absorbed or scattered by other items beyond the object. Thus, no objects or persons should be within six to fifteen feet of the apparatus otherwise the person or object will be unnecessarily exposed to the x-rays and may even appear in the image.
Thus, there is a need for an apparatus that would overcome the disadvantages of prior art x-ray systems and allow for the detection of concealed objects in the same plane as the background. There is also a need for a way to control x-rays that are not scattered or absorbed by the object to protect other persons from unnecessary exposure to the x-rays and to prevent image degradation.